The present invention relates to a new aluminium-titanium alloy of high specular reflectivity, reflective coatings comprising the alloy and mirrors and components including the coating.
The invention relates more particularly to the design and manufacture of special mirrors. Special mirrors are mirrors whose optical characteristics must lie within a well-defined range and which must be preserved in their entirety, even if they are subjected to severe constraints. Examples of severe constraints that are not limiting on the invention are particularly corrosive atmospheres, such as moist atmospheres, with or without condensation, in the presence of chemical agents such as sodium chloride, for example. Even more particularly, but without this being limiting on the invention, the invention relates to mirrors made by depositing a layer of metal onto a plastics material or composite material support; in the latter case, the interface between the reflective coating and the substrate, which is usually a polymer, is particularly sensitive to the environment and is the site of irreversible deterioration.
Special mirrors to resist aggressive environments are often made by depositing a layer of metal onto a support providing an adequate structure. The metal layer is then protected against mechanical, chemical or other aggression by a transparent protective film. One non-limiting example of a protective film is a metal oxide or oxynitride, although for economic reasons it is very often a polymer.
A mirror of this kind therefore includes two interfaces, a substrate/reflective coating interface and a reflective coating/protective film interface; these are weak points because they can be the site of separation leading to tarnishing, blistering and, in all cases, serious deterioration of the mirror.
In the prior art, the special mirrors described above are generally made by depositing a metal referred to as xe2x80x9cfrom the platinum minexe2x80x9d, i.e. from group VIII of the periodic table of the elements, and more particularly rhodium, ruthenium or palladium. These metals are of acceptable reflectivity, close to 60%. They are not sensitive to tarnishing and have good resistance to mechanical aggression because of their high hardness. However, this technique has major disadvantages connected with the difficulty of making coatings with these metals and their very high cost.
Some manufacturers make mirrors by depositing pure aluminium or silver protected by a transparent polymer onto a metal, polymer or ceramic substrate. Such mirrors have excellent optical properties but cannot be used in an aggressive atmosphere as they deteriorate very quickly through tarnishing, blistering and scaling of the coating.
A layer of titanium has been used, associated or not with a layer of chromium, as described in FR 2768096, but these structures deteriorate quickly in an aggressive environment, to the point that FR 2768096 recommends depositing the layers onto the rear face of a transparent support, indicating that it is then particularly well protected from mechanical and chemical influences.
Alloys of titanium and aluminium are known in the art. They are used to confer mechanical properties on the titanium; these alloys have a low aluminium content and generally contain copper.
One object of the present invention is to provide a metal alloy simultaneously having the properties of being a good reflector and of forming very strong chemical bonds with the substrates, in particular polymer substrates, so that the two interfaces mentioned above are resistant to most forms of atmospheric aggression, whether of an industrial or saline nature.
Another object of the invention is to provide metal alloys that are intrinsically resistant to corrosion and therefore to tarnishing.
A further object of the invention is to provide an alloy forming chemical bonds which are not degraded by variations in temperature over a wide range of use with the substrates, in particular with polymer substrates.
It is also an object of the invention to provide reflective coatings at reduced cost.
The above objects, together with others that will become apparent on reading the following description, are satisfied by the present invention which provides a microcrystalline metal alloy based on aluminium and titanium whose aluminium content is from 80 to 90 atomic percent, whose titanium content is from 10 to 20 atomic percent, that is not in thermodynamic equilibrium, and that is therefore resistant to oxidation and to corrosion, and that simultaneously has a remarkable capacity for adhesion to polymer materials.
In the context of the invention, the term xe2x80x9cmicrocrystallinexe2x80x9d refers to a crystal having a size less than approximately 1 xcexcm. According to the invention, a microcrystalline alloy is therefore constituted of crystals having a size less than approximately 1 xcexcm.
The performance of the metal alloy of the invention, in particular with regard to its reflectivity, is improved if the alloy is purer. However, up to 2 atomic percent of impurities can be tolerated without significantly affecting the performance of the alloy.
Consequently, in one preferred embodiment of the invention the metal alloy comprises not more than approximately 2 atomic percent of impurities.
In a preferred embodiment of the invention the metal alloy has an aluminium content from 84 to 87 atomic percent and a a titanium content from 13 to 16 atomic percent.
It has been necessary to overcome a major prejudice to arrive at the present invention because it is well known in the art that adding addition elements to aluminium will normally significantly reduce its reflectivity.
Alloys of titanium and aluminium are known in the art but these are generally alloys with a low content of aluminium and generally contain copper and are used to confer mechanical properties on the titanium.
Aluminium alloys with a low content of titanium have been described, generally associated with equivalent quantities of manganese and magnesium, the titanium content not exceeding one atom percent. Intended for the fabrication of hollow bodies under pressure, such alloys are totally outside the field of application and the scope of the invention.
The publication xe2x80x9cBinary Alloy Phase Diagramsxe2x80x9d by T. B. Massalski, vol 1, pages 175-176, discloses alloys of aluminium and titanium whose aluminium content is from 80 to 90 atomic percent and whose titanium content is from 10 to 20 atomic percent. These alloys, which are outside the scope of the present invention, are in thermodynamic equilibrium and are not microcrystalline. They do not have the required resistance to oxidation and to corrosion and do not have a significant capacity for adhesion to polymer materials.
The present invention also provides a reflective coating consisting of a layer of metal alloy according to the invention described above covered with a protective film.
The metal alloy is an alloy based on aluminium and microcrystalline titanium which is not in thermodynamic equilibrium, whose aluminium content is from 80 to 90 atomic percent and whose titanium content is from 10 to 20 atomic percent.
In a preferred embodiment of the invention the reflective coating consists of a layer of metal alloy covered with a transparent protective film, and the metal alloy is an alloy based on aluminium and titanium whose aluminium content is from 80 to 90 atomic percent, whose titanium content is from 10 to 20 atomic percent and whose impurities content is at most equal to approximately 2 atomic percent.
In the reflective coatings of the present invention the protective film is usually a polymer material.
The thickness of the metal alloy layer is generally from 0.01 to 5 xcexcm.
Below 0.01 xcexcm the coating is quasi-transparent. Beyond 5 xcexcm the reflectivity is degraded, although the other properties are not affected.
The thickness of the layer of metal alloy is preferably from 0.01 to 3 xcexcm and more preferably from 0.01 to 0.5 xcexcm.
The present invention also provides a mirror comprising a substrate supporting a reflective coating as described above.
The substrate is generally a polymer material.
To obtain a special mirror having a specular reflectivity at least equal to approximately 65%, good resistance to corrosion and to oxidation, it is necessary to choose a reflective coating consisting of a layer of metal alloy covered with a transparent protective film, characterized in that the metal alloy is an alloy that is based on aluminium and microcrystalline titanium, that is not in thermodynamic equilibrium, whose aluminium content is from 80 to 90 atomic percent, whose titanium content is from 10 to 20 atomic percent, and whose impurities content is at most equal to approximately 2 atomic percent, and in which the thickness of the alloy layer is from 0.01 to 3 xcexcm.
When the reflective coatings of the invention are deposited on a polymer substrate and protected by a transparent protective film, the combination can be exposed to a corrosive atmosphere such as that of the salt mist test of ISO standard 9227 without affecting the reflectivity or causing defects to appear at the interfaces.
The aluminium/titanium alloy according to the invention can be deposited by any appropriate means; however, vacuum deposition techniques such as cathode sputtering in its various forms, evaporation and co-evaporation under certain conditions (which can readily be determined by the skilled person in order to obtain microcrystals) represent means that are particularly indicated for making such materials in the form of thin layers.
The mirrors of the invention, in particular the special mirrors, can be parts of components such as automobile rear view mirrors or billboards.
The alloy in accordance with the invention has a very high affinity for and a very high adhesion to polymers.
The corrosion resistance is also high because the reflecting face of the mirror is in direct contact with the atmospheric agents and is not protected from oxidation.
Nippon Jidosha patent JP 61133902 describes a mirror consisting of a glass substrate on which are successively deposited a transparent layer of TiO2 and a reflective layer of an aluminium alloy, for example aluminium titanium alloy. The aim is to make a coloured mirror, the colour being produced by luminous interference within the TiO2 layer. The titanium dioxide is the most important component of the coloured mirror. Note that the reflecting layer of aluminium alloy is on the rear part of the glass relative to the light rays; its reflecting face is therefore protected from oxidation. What is more, the aluminium alloy of the above patent, which is in contact with a ceramic, unlike that of the present invention, does not have to adhere to or have an affinity for a polymer, ceramics being entirely different from polymers on the physical-chemical plane.